Couplers play an important role in network measurement systems. In addition to providing connections between a network measurement system and a device under test (DUT), couplers provide the important function of separating incident and reflected waves for network measurements. A measure of this separation between incident and reflected waves, referred to as directivity, influences the measurement accuracy of the network measurement system. Higher directivity generally increases measurement accuracy. Directivity is an especially important performance measure of a coupler when measuring DUTs that have impedance-matched ports and attenuation losses that result in reflected waves that have low magnitudes.
One known type of coupler, shown in the schematic diagram of FIG. 1, is a directional bridge. The directional bridge is commonly used in network measurement systems, such as scalar and vector network analyzers. While the directional bridge provides excellent low frequency performance, important performance parameters of the directional bridge such as directivity, coupling, insertion loss, and isolation, degrade above approximately nine gigahertz, as shown in the response plot of FIG. 2. Thus, the directional bridge, which has excellent performance at low operating frequencies, has poor performance at high operating frequencies.
In contrast to the directional bridge, wherein various signal paths have physical connections to each other, a proximity coupler includes coupled transmission lines that are physically separated. Physical separation between the coupled transmission lines causes this type of coupler to have a low-frequency operating limit of approximately ten megahertz. While proximity couplers generally have poor performance at low frequencies, proximity couplers can be designed to have high-frequency operating limits that exceed twenty gigahertz.
In many types of network measurement systems, there is a need for couplers that have high directivity and relatively flat coupling over a wide frequency range. For example, a network analyzer may have an operating frequency range that spans from several hundred kilohertz at the lower operating frequency limit, to twenty gigahertz at the upper operating frequency limit. To achieve such a wide operating frequency range, commercially available network measurement systems typically use a proximity coupler and a directional bridge connected in a parallel, switched arrangement, wherein a switch selects between the two different types of couplers according to the operating frequency of the network measurement system. However, this switched arrangement is cumbersome since it requires the two couplers, the switch, and a control signal to set the position of the switch. The switch also has the disadvantage of introducing power losses at the test ports of the network measurement system, which can reduce the measurement sensitivity of the network measurement system within which the switched coupler arrangement is included.